The instant invention relates to permanent magnet A.C. machines, and, more particularly, to such machines having a two-pole configuration.
Rare earth cobalt magnets, as they are presently available, provide flux densities at the maximum energy point of around 0.4 to 0.6 Tesla. Since the maximum energy point also represents the lowest magnet volume for the magnetic energy required, one would like to operate the magnets close to the maximum energy point for maximum magnet utilization. In conventional magnet arrangements for two-pole permanent magnet machines, maximum magnet utilization results in a maximum achievable flux density in the annular air-gap surrounding a cylindrical rotor of less than 57% of the operating flux density of the permanent magnets. The precise percentage of maximum achievable flux density in the air-gap would depend entirely upon how much leakage flux exists in the rotor construction. The resulting low flux density in the air-gap requires a large machine for a given output power requirement, making the machine noncompetitive with wirewound machines, with respect to material utilization and machine efficiency, because the large machine volume required for a given power output more than compensates for the low loss densities achievable with permanent magnet machines.
Two representative prior art permanent magnet machines are described in British Pat. Nos. 1,176,550 and 1,437,348. Pat. No. 1,176,550 illustrates a machine in which nonmagnetic material is used to plug the spaces between magnets and pole pieces. Pat. No. 1,437,348 discloses a machine in which magnetic discs connect magnets and pole pieces. Each of these configurations experiences considerable leakage of magnetic flux, significantly reducing machine output from the theoretical maximum for a fixed magnet volume.